Pickling of metal powders is a known method of reducing surface oxides to increase formability of water atomized metal powders. However, high nickel and chromium alloys typically have required strong solutions for pickling such as salt baths, hydrofluoric acid or mixtures containing hydrofluoric acid. Previous pickling, as disclosed in U.S. Pat. No. 4,818,482 requires a first step rinse with an acid mixture of nitric acid and hydrofluoric acid. This hydrofluoric acid is hazardous, corrosive and requires special and costly safety procedures. Furthermore, neutralization and/or disposal of liquors and fume emissions are subject to very stringent control regulations. The best results were achieved by pickling with nitric and hydrofluoric acid, rinsing with water, pickling in an oxidizing alkaline bath and pickling again with nitric and hydrofluoric acid. Although effective, the above process contains several steps and relies upon undesirable hydrofluoric acid during two pickling steps of the operation.
It is an object of this invention to provide a hydrofluoric acid free method of pickling corrosion resistant powders.
It is another object of this invention to provide an effective method of removing surface oxides from metal powders.
It is another of object of this invention to provide a method of forming water atomized corrosion resistant powders into objects of sufficient green strength for further handling.